1. Field of the Invention
This invention relates to a method and a system for ultrasonic detection and imaging of small defects inside or at the surface of an object by an improved version of the Synthetic Aperture Focusing Technique, and particularly to such method where ultrasound is generated by a laser and detected by either a contact ultrasonic transducer or a laser interferometer.
2. Description of Prior Art
Ultrasound is a well-recognized technique for finding defects or discontinuities in objects. Ultrasound provides not only information on the presence of such discontinuities, but also an indication on their depth, deduced from the arrival time of the echoes and the knowledge of the elastic wave velocity. By scanning the surface with a piezoelectric transducer, the object can be mapped out throughout its entire volume and the information displayed as B-scans or C-scans. B-scans are planar cuts through the material and indicate directly the depth of the discontinuities that are found. C-scans are more like views from the surface and provide depth information by using a color or gray scale code. The coding may be associated either to the arrival time of echoes or their amplitude. Ultrasound can also be used to find flaws at the surface of objects by using waves propagating at their surface (surface or Rayleigh waves) or when the object is a thin plate by using Lamb waves.
High-resolution imaging and a better definition of the defects are obtained by focusing ultrasound with acoustic lenses or curved transducers. Alternatively, instead of physically focusing ultrasound inside the object (or at its surface), a numerical focusing technique, called Synthetic Aperture Focusing technique (SAFT), can be advantageously used. SAFT allows a lens with a very large effective aperture to be realized numerically , which leads in turn to improved resolution. SAFT has also the advantage of being more easily applicable to objects with complex shapes, once the object contour is known and does not require the realization of a special transducer adapted to the shape of the object. SAFT is implemented by providing a small ultrasonic source at the object surface with a focused transducer and scanning this source over the surface. As shown in FIG. 1a, detection is usually performed at the same location as generation (other schemes are possible) resulting in a 2-D array of signals. SAFT performs a summation of N signals shifted in time and taken from the measurement grid within a given aperture (the synthetic aperture). The time shift of each signal is a function of the point where the signal is collected and the point at a depth z where the presence of a defect is to be determined. The coherent summation increases the SNR for defect detection by the factor .sqroot.N. While maintaining the axial or depth resolution .DELTA.z, the SAFT processing improves the lateral resolution .DELTA.x. It can be shown that the depth and lateral resolutions for defect sizing are given by: ##EQU1## where v is the ultrasonic wave velocity, .DELTA.t is the ultrasonic pulse duration and a is the dimension of the synthetic aperture. Examples of implementation of SAFT can be found in U.S. Pat. Nos. 4,841,489 (Osaki et al.) and 5,465,722 (Fort et al.). See also the discussions in S. R. Doctor, T. E. Hall, L. D. Reid, "SAFT--the evolution of a signal processing technology for ultrasonic testing", NDT International, 19, 163 (1986) and J. A. Seydel, "Ultrasonic synthetic aperture focusing techniques in NDT", in Research Techniques in Nondestructive Testing Vol. 6, R. S. Sharpe, Ed. NY: Academic, 1983.
SAFT can also be advantageously applied when using lasers for the generation and detection of ultrasound (a technique called laser-ultrasonics). Laser-ultrasonics uses one laser with a short pulse for generation and another one, long pulse or continuous, coupled to an optical interferometer for detection (see FIG. 1b). Details about laser-ultrasonics can be found in C. B. Scruby, L. E. Drain, "Laser-ultrasonics: techniques and applications", Adam Hilger, Bristol, UK 1990 and J.-P. Monchalin, "Optical detection of ultrasound," IEEE Trans. Ultrason. Ferroelectr. Freq. Control, 33, 485 (1986). By relying on optics for providing the transduction of ultrasound, laser-ultrasonics brings practical solutions for testing at a large standoff distance, for inspecting moving parts on production lines and inspecting in hostile environments (for example, see J.-P. Monchalin et al., "Laser-Ultrasonics: From the Laboratory to the Shop Floor", Advanced Performance Materials, vol. 5, pp. 7-23, 1998). Generation of ultrasound can be performed either in the ablation or thermoelastic regime. In the first case, a sufficiently strong laser pulse provides vaporization or ablation of the surface. The recoil effect following material ejection off the surface produces strong longitudinal wave emission. In the thermoelastic regime, the emission pattern depends on the penetration of light below the surface, which could range typically from microns to hundreds of microns in the case of polymers to practically no penetration in the case of metals. Penetration produces a buried source and a constraining effect that also favors longitudinal ultrasonic emission. In all cases, shear waves are also emitted. When the source is small (smaller than the acoustic wavelength) a complex pattern of emission is obtained, having in some cases several emission lobes. It should be noted that with laser generation, the ultrasonic source is located at the surface of the part and follows automatically the contour. Regarding optical detection, the small phase or frequency shift in the scattered light induced by the ultrasonic surface motion is detected by an interferometric system. For applications where the inspected part is scanned or is moving, a detection scheme that is independent of the speckle or integrates over the whole speckle field is needed. A passive approach based on time-delay interferometry may be used or one can rely on an active one using nonlinear optics for wavefront adaptation. Examples include those discussed in U.S. Pat. Nos. 4,659,224 (Monchalin), 4,966,459 (Monchalin), 5,137,361 (Heon et al.), 5,131,748 (Monchalin et al.) and 5,680,212 (Blouin et al.).
For the detection of small defects, laser-ultrasonics has similar limitations to conventional piezoelectric-based ultrasonics, caused by the wave nature of the interrogation and diffraction effects. The spatial resolution of laser-ultrasonics depends upon the spot sizes of the generation and detection lasers and may be inadequate for detecting small and deep flaws. The use of a broad laser spot to produce an ultrasonic beam with little divergence gives a resolution essentially limited by the spot size. In the opposite case, focusing the laser beam to a small laser spot yields a strongly diverging acoustic wave, leading also to poor resolution. Similarly to conventional ultrasonics, SAFT can be used in conjunction with laser-ultrasonics to improve resolution. Examples of implementation can be found in U.S. Pat. Nos. 5,615,675 (O'Donnell et al.) and 5,801,312 (Lorraine et al.). However the technique described in these two patents presents several difficulties which limit their applicability. A first difficulty originates from the fact that lasers have usually relatively low repetition rates, usually much lower than piezoelectric transducers, which makes data acquisition time very long, so a way to minimize data acquisition duration while maintaining adequate resolution is desirable. A second difficulty is the long time taken by SAFT processing with the time domain approach used in these two patents. This approach is the one that has been explained above. This time domain approach, while straightforward in its principle and implementation, is not very efficient and is very computation intensive. A simple analysis reveals that the processing time scales as n.sup.5 for a cubic data block, with n being the number of data points along each axis. A third difficulty originates from the ultrasonic pulse produced by laser generation. This pulse has a unipolar shape so it does not provide destructive interference at locations without defects, resulting in a broad background around discontinuities. As indicated in the Lorraine's patent, this problem was solved by filtering the low frequency components of the waveform data to restore a bipolar pulse shape suitable for use with SAFT. Considering that high spatial resolution relates to a short pulse duration (see equation 1) or a large frequency bandwidth, filtering the low frequency components does not appear to be optimal.
To solve the second difficulty just mentioned, i.e. to improve computational efficiency, SAFT can be implemented in the frequency domain where advantage is taken of the fast Fourier transform (FFT) algorithms. Data processing is performed in the 3-D Fourier space using the angular spectrum approach of the scalar diffraction theory. The use of this method has been reported by K. Mayer, R. Marklein, K. J. Langenberg and T. Kreutter, "Three-dimensional imaging system based on Fourier transform synthetic aperture focusing technique", Ultrasonics 28, 241 (1990) and L. J. Busse, "Three-dimensional imaging using a frequency-domain synthetic aperture focusing technique", IEEE Transactions UFFC 39, 174 (1992). Even with improved computation efficiency, the known frequency-domain method does not provide a clue on how to get optimum resolution. A way to control the aperture size is also missing, which is significant since the strength of the ultrasonic wave and the detection sensitivity both decrease as the lateral distance between the sampling point and the observation point increases and adding contributions from highly offset points contributes more noise than signal. This is straightforward in the time-domain SAFT, but not in the frequency-domain SAFT. This control is particularly important for laser-ultrasonics and in practice, the total opening angle of the synthetic aperture is expected to be limited to roughly 60.degree. when longitudinal waves produced by an ablation or constrained source mechanism are used, which means a.about.z in equation (1). When shear waves are used, the aperture should be annular. Also, previous art related to SAFT processing does not teach how to minimize the number of sampling points in order to minimize both data collection and processing durations, while keeping adequate resolution.
It is an object of the present invention to provide a method that alleviates the afore-mentioned limitations in the prior art.